As a low-pollution coating without using an organic solvent, there has been a growing demand for a powder coating in fields such as automobile parts, electric appliances, furniture, machine tool, business equipment, toys and the like. Coating with the powder coating causes low pollution. In addition, sufficient thickness of the coating film can be achieved by one time application, and it is not necessary to apply coating as many times as in an example using a conventional solvent-type coating. That is, a time period for coating can be shortened. Moreover, as the solvent is not contained in the coating, the powder coating is advantageous in that a pinhole will not be created in the coating film.
In the powder coating with a property as described above, a coating film property is excellent if a metal pigment such as metal flake is not contained, presenting no problem. On the other hand, if a flake metal pigment is contained, following problems will arise.
Examples of a conventional method of metallic coating with a powder coating include: a melt blending method in which a flake pigment is sufficiently mixed and kneaded with a resin or a coloring pigment in advance using a melting method, and thereafter a resultant substance is pulverized by crushing or the like; a dry blending method in which resin powder and a flake pigment are mixed and applied; and a bonded method using resin powder having its surface adhered to a flake pigment (for example, see the specifications of Japanese Patent Laying-Open No. 51-137725, Japanese Patent Laying-Open No. 9-71734, U.S. Pat. No. 4,138,511, and the like).
In the melt blending method, the flake pigment tends to be deformed during the step of mixing and kneading or the subsequent step of controlling a resin powder particle size by crushing, and excellent metallic feel cannot be obtained after coating. In addition, when an aluminum flake pigment is used as the flake pigment, an active surface of aluminum is exposed during crushing, and possibility of ignition, dust explosion or the like will be higher.
In the dry blending method, deformation of the flake pigment is relatively unlikely. On the other hand, as the pigment needs to bear electrical charges during coating, a surface should be coated with a resin in advance, if a metal pigment such as the aluminum flake pigment is used. In addition, as the pigment and the resin powder have different charged rate respectively, a separation phenomenon between the resin powder and the metal pigment takes place during coating, resulting in lower performance in terms of design of the coating film. Further, a ratio of content of the pigment varies before and after application of the powder coating. Therefore, if the coating is recovered for reuse, color tone is altered. This means that recycle of the coating is virtually impossible.
In addition, in the dry blending method, performance in terms of design is impaired even with the aluminum flake pigment that has been coated with resin in advance. Specifically, when powder-coating of the powder coating composition is carried out by using a corona gun, the aluminum flake pigment adheres to a gun nozzle due to static electricity. After the adhered aluminum flake pigment grows to some extent, it is separated, which results in such a spitting phenomenon that an aluminum mass adheres to a plate, i.e., production of a seeding.
The bonded method includes a method of adhering the flake pigment to a resin powder surface with a brush polisher, and a method of transferring and adhering metal flake to the resin powder by causing the resin powder to come in contact with a distributing medium such as an alumina ball covered with the metal flake, and the like. In such bonded methods, the flake pigment and the resin are pressed and adhered by physical stress. Accordingly, deformation of the flake pigment tends to occur, and excellent metallic feel cannot be obtained. In addition, adhesion is not strong. This fact is advantageous in that bonding (blocking) among resin powders is less likely, whereas free particles of the flake pigment that do not adhere to the resin powder considerably remain. If an amount of the free flake pigment increases, a blending ratio of the resin to the flake pigment is varied due to a difference in adhesion efficiency when the coating is recovered for reuse, whereby the coating cannot be reused after it is recovered as in the case of the dry blending. Moreover, if the metal pigment such as the aluminum flake pigment is used, the possibility of ignition or dust explosion becomes higher. Weak adhesion between the resin powder and the flake pigment is noticeable particularly when the flake pigment has a large particle size. Excellent brilliance and high brightness only achieved with the use of such a flake pigment have been difficult to achieve with the bonded aluminum obtained by using these methods.
From the viewpoint described above, development of a powder coating composition containing a flake pigment having excellent recyclability and coating property as well as improved metallic feel, brilliance and brightness of the coating film has strongly been demanded. Such a powder coating composition, however, has not yet been developed.